The present invention relates generally to electric machines and, more particularly, to implementations for sensorless electric machines that have skewed rotor sections.
One general application for electric machines, and interior permanent magnet (IPM) machines, in particular, is for use in underground mining vehicles, wherein typically electric wheel motors (e.g., IPM) are connected to the wheels via a gearbox. This application typically requires very high torque at low speeds and yet maintenance of the rated power over a very wide speed range (e.g., on the order of 15:1).
IPM machines unfortunately suffer from both a manufacturing and a technical (i.e., electro-mechanical) shortcoming. With IPM machines, permanent magnets typically are inserted into slots in the rotor structure and pushed entirely through the entire slot depth in order to fill the entire stack length. Due to small clearances between the magnets and the slots in the laminations, and the unevenness of such slots along the entire length, the magnets and/or laminations may be damaged during this insertion process.
Further, depending on their magnitude, torque “ripple”, or torque oscillations, of the IPM or with Synchronous Reluctance machines, may result in damage to the rotor, the gearbox, and/or the mechanical system(s) connected to the electric machines (due to fatigue or excessive torque). Additionally, the frequency of the torque ripple might excite resonant modes of the mechanical system(s), further posing an additional threat to the electric machines and/or surrounding systems.
Various attempts at reducing torque ripple have included modifying the stator, via stator skewing with a continuous skewing arrangement. This methodology suffers from an undesirable increase in manufacturing cost and complexity. For example, this can cause an additional complexity with the inserting of coils into the slots. Another countervailing trend in reducing torque ripple is using an odd number of stator slots per pole pair. While this method has proven effective in helping reduce torque ripple, it suffers from the undesirable tradeoff of increasing core losses, which, in turn, may harm efficiency.
Further with an electric machine, be it at the IPM machine, permanent magnet (PM) assisted synchronous reluctance machine, or the synchronous reluctance machine, position is a critical informational element for torque control. Typically, an encoder, tachometer, or resolver is used with electric machines as the position sensor.
However, the position sensor (e.g., encoder) along with its cabling and interface electronics contributes a significant portion of the motor drive system cost and overall complexity and is often a major reliability concern. Since the advent of the high frequency injection method for zero frequency encoderless control, encoderless controls have seen great improvements but none have found success in recovering the full, or near full, torque capability of the machine. This is due to loss of small signal saliency at high-load levels for the machine.
Accordingly, there is an ongoing need for improving on current electric machine technologies and/or manufacturing thereof that address at least one of complexity, cost, efficiency, and/or performance without some of the current tradeoffs encountered with current methodologies.